The present invention relates to an amplifying circuit, and to means, which enables a differential amplifier to exhibit a zero common mode gain or a variable common mode gain.
Single stage differential amplifier circuits are used in many electronic applications, such as programmable logic arrays. For programmable logic arrays differential amplifier circuits are designed to vary the common-mode gain and common-mode rejection ratio utilizing more than one amplifier stage and/or with additional complex electronic circuitry. It would be desirable therefore to provide a circuit that improves the features of differential amplifiers with the use of a minimum amount of components.
In accordance with the present invention, there is disclosed a differential amplifier circuit connected to an impedance transport circuit which is capable of transporting an AC input impedance to an output impedance and invert its polarity thereby enabling the common-mode gain of the differential amplifier to be varied. This impedance transport circuit utilizes two MOS transistors and an Op-Amp that causes the drain voltages of the transistors to be equal